You could use a push-pull pot--I'd put it on the tone control--to add the bridge pickup to whatever's selected on the 5-way. You'd get a few redundant positions--the bridge-only position (#1) and the bridge-and middle (#2) wouldn't change, and the middle position (#3) with the push-pull activated would be the same as #2--but pull the pot on the neck-and-middle position (#4) and you get all three pickups on, and the pull on #5 (neck pickup) gives you neck-and-bridge together. I've got a Carvin DC145 (two humbuckers with coil splitters plus a single-coil in the middle) wired up that way, and it works great. There is some loss of level with the coil-splits on the Carvin, and I understand the Brent Mason doesn't do that--just one of the reasons I want to try one!
yankeebulldog--is that platinum metallic or frost blue? Looks great!